Inkjet printing is accomplished by ejecting ink from a nozzle toward paper or another print medium. The ink is driven from the nozzle toward the medium in a variety of ways. For example, in electrostatic printing, the ink is driven by an electrostatic field. Another inkjet printing procedure, known as squeeze tube, employs a piezoelectric element in the ink nozzle. Electrically-caused distortions of the piezoelectric element pump the ink through the nozzle and toward the print medium. In still another inkjet printing procedure, known as thermal or bubble inkjet printing, the ink is driven from the nozzle toward the print medium by the formation of an expanding vapor phase bubble in the nozzle. These various printing methods are described in “Output Hard Copy Devices,” edited by Durbeck and Sherr, Academic Press, 1988 (see particularly chapter 13, entitled “Ink Jet Printing”).
The composition of an inkjet ink is traditionally comprised of deionized water, a water-soluble organic solvent, and a colorant. The colorant may be a soluble dye or insoluble pigment. Pigment based inks may have some disadvantages as compared to dye based inks. For example, due to the insolubility of pigments in water, polymeric dispersants and/or surfactants are typically added to improve the dispersibility of the pigment. The addition of a polymeric dispersant can increase the viscosity of an ink, and a viscous ink may be more difficult to jet in a printhead.
To potentially avoid the use of or to reduce the amount of polymeric dispersants used in pigmented ink compositions, pigments can be modified by direct oxidation to generate carboxy groups (COOH) on the surface of the pigment that can make the pigment self-dispersible in aqueous compositions. Such pigments exhibit increased electrostatic stability. However, the electrostatic stability can be sensitive to other characteristics of the ink composition. In particular, electrostatic stability can be very sensitive to the pH and electrolyte concentration. Accordingly, self-dispersing pigments can be prone to coagulation especially within the nozzle of an inkjet print system. Furthermore, such self-dispersing pigments may not be readily soluble with the other components of an ink composition such as humectants and binders.
Thus, the need exists for inks containing self-dispersing pigments with enhanced stability and solubility in the ink composition.